We propose to continue our study of the cellular and molecular basis of selective transport of IgG immunoglobulins across the epithelial cells in the small intestine of the neonatal rat. Major emphasis is placed on the characterization of the IgG receptors isolated from intestinal brush borders by affinity chromatography. SDS-gel electrophoresis will be used to analyze the receptor to determine (a) molecular weight, (b) carbohydrate and lipid content, and (c) subunit structure. Gel filtration will be used to estimate valency of the receptor for IgG. Degradative enzymes will be used to determine the relative importance of carbohydrate and protein for receptor activity. if the receptor can be isolated to a suitable degree of purity, the structure of the receptor will be examined with negative stains for electron microscopy. The purified receptor will also be used as an antigen for production of specific antibodies to be employed in morphological experiments on the cellular and tissue distribution of receptor sites. Morphological studies which will be undertaken include (a) the effect of IgG concentration at the luminal cell surface on initiation of pinocytosis, (b) role of specific antibody in the passive transport of antigens to the circulation, (c) the distribution and movement of IgG receptors on the albuminal surface of cells, and (d) the possible distribution of antigenically similar IgG receptors in other tissues. Methods to be employed include the use of peroxidase conjugates of normal IgG and of anti-receptor antibody as specific tracers for light and electron microscopy.